Water is supplied to homes by pipes that run underground, where the temperature is relatively cool and constant. From there, pipes are routed through the floor and up the walls to the fixtures. Alternately, the pipes can run through the attic instead of the floor. This is often done when re-piping a home to replace old leaky pipes, since running replacement pipes through an attic is easier than through the floor. Running new pipes through an existing concrete floor slab is impractical.
On hot days, the attic heats attic pipes. Cold water faucets then produce hot water until the heated water in the attic pipes runs out. This water can be so hot that it is unsafe, unsuitable, or uncomfortable for cold tap water uses, such as washing hands and delicate clothing. Insulating the pipes gives little benefit because water in the pipes is usually stationary, so it absorbs heat gradually through the insulation until it reaches the attic temperature. Installing a radiant barrier in the roof or attic is expensive and only partly effective. A user can run the cold water until the hot water from the attic pipes runs out, but this wastes water and time.